Combustion is the rapid combination of oxygen with a fuel, such as natural
gas, resulting in the release of heat. Most fuels contain carbon and hydrogen,
and the oxygen usually comes from air. Combustion generally consists of
the following overall reactions:

Stoichiometric
or perfect combustion is obtained by mixing and burning exactly the correct
proportions of fuel and oxygen so that no oxygen remains at the end of
the reaction. If too much oxygen is supplied, the mixture is lean and
the reaction is oxidizing. This results in a flame that is relatively
shorter.

If too much fuel is supplied,
the mixture is rich and the reaction is reducing. This typically results
in a flame that is relatively longer and sometimes smoky. Most industrial
burners are supplied with some excess air to mitigate the formation of
unburned hydrocarbons, carbon monoxide and particulate matter.

Air Requirements for Perfect Combustion

The following table lists the proper proportions for perfect combustion
of some gaseous fuels:

The products of full combustion
for natural gas primarily consist of carbon dioxide, water vapor, nitrogen
and excess oxygen. The oxygen supply for combustion usually comes from
air. Because air contains a large amount of nitrogen, the required volume
of air is much larger than the required volume of oxygen. The nitrogen
absorbs some of the heat released in the reaction resulting in a much
lower flame temperature compared to a reaction in pure oxygen.

Primary air
is the air that is mixed with the fuel at or in the burner. Secondary
air is usually the air that is brought in around the burner.
Tertiary air is usually the air that is introduced
downstream of the secondary air or through other openings in the combustion
furnace.

Good combustion requires:

the proper proportioning
of fuel and air

thorough mixing of the
fuel and air

initial and sustained
ignition of the mixture

Flame Temperature

The following
graph illustrates how the theoretical flame temperature varies with the
air/gas ratio:

click on image for a larger view

Heat
Available from Natural Gas

When a perfect mixture of a
fuel and air, originally at a specified reference temperature (usually
60°F) is ignited and then cooled to the reference temperature, the
total heat released is termed the higher or gross heating value. The lower
or net heating value is the gross heating value minus the heat released
by the condensation of the water vapor in the combustion products.

For natural gas, which is primarily
composed of methane, the higher heating value
is typically in the range of 1000 to 1050 Btu/ standard cubic foot (scf).
The chemical composition of natural gas varies slightly depending upon
the location in North America. The exact value for your fuel can be obtained
from your local natural gas distribution utility.

Perfect combustion of natural
gas requires approximately 9.5 scf of air per scf of natural gas and results
in approximately 10.5 scf of products of combustion. The values vary somewhat
depending upon the natural gas composition.